(1) Microbial Pesticides
The most widely used microbial pesticides are derived from the bacterium Bacillus thuringiensis. This bacterial agent is used to control a wide range of leaf-eating caterpillars, and mosquito Bacillus thuringiensis produces a proteinaceous paraspore or crystal which is toxic upon ingestion by a susceptible insect host. For example, B. thuringiensis var. kurstaki HD-1 produces a crystal called a delta toxin which is toxic to the larvae of a number of lepidopteran insects. The cloning and expression of this B.t. crystal protein gene in Escherichia coli has been described in the published literature (Schnepf, H. E. and Whitely, H. R. [1981]Proc. Natl. Acad. Sci. USA 78:2893-2897). U.S. Pat. No. 4,448,885 and U.S. Pat. No. 4,467,036 both disclose the expression of B.t. crystal protein in E. coli.
(2) Lepidopteran Pests
The beet armyworm (BAW) Spodoptera exiqua is a widely distributed noctuid moth that attacks a broad range of field and vegetable crops. This economically important species originated in Asia, but is now found in many parts of the world including the United States.
The plants attacked by BAW include beets, peanuts, alfalfa, lettuce, asparagus, tomatoes, potatoes, corn, onions, peas, cotton, citrus, mallow, and even certain wild grasses. It is also a pest on ornamentals and floriculture crops, such as carnations and chrysanthemums. Larvae will feed on the leaves, stems, buds, and sometimes the roots of host plants. Heavy infestations can lead to complete defoliation of fields of a crop, such as table beets.
The female oviposits egg masses of about 80 eggs on the host plant foliage. These egg masses are covered with hairs and scales from the body of the female. An average of 500 g to 600 eggs may be deposited over a 4 to 10 day period Larvae hatch in 2 to 5 days and begin feeding on the foliage. Young larvae will feed in growing tips of the plant and developing buds, while older larvae are less discriminating, feeding on older foliage as well. The five larval instars take about 3 weeks to complete, at which time the mature larva drops to the ground and pupates in the soil. In the warmer parts of its range the BAW passes through four generations per year.
This species is generally considered to be difficult to control in various crop situations. Methomyl (Lannate) is commonly used to control this pest in lettuce and other field crops. However, resistance to methomyl has been reported in populations exposed to heavy use of this chemical (Yoshida and Parella [1987]). Consequently, there is a need to develop alternative control strategies for this important pest.
Another aspect of the use of broad spectrum materials like Lannate for BAW control is secondary pest outbreaks. This is the disruptive influence of a non-selective chemical on natural control agents of other pests in a given crop. In tomatoes, chrysanthemums, and other groups, where leaf miners can be a problem, the use of Lannate severely depresses populations of the natural enemies of the leafminers. With removal of leafminer parasites, the leafminers can build to very high population levels and cause severe damage.
The discovery and use of a novel Bacillus thuringiensis isolate with good activity against BAW is a distinct improvement in the control of this lepidopteran pest.